In recent years, advances in technology, as well as ever evolving tastes in style, have led to substantial changes in the design of automobiles. Electric motors (or electric machines) are finding an increasing number of applications in the automotive industry due to the electrification of the automotive drive system. Electric and/or hybrid vehicles utilize electric motors as either primary or supplemental torque sources in the automotive drive system. These electric motors are expected to function over extreme operating conditions for an extended period of time with high reliability. However, over time, the operating stresses applied to the electric motor may degrade the condition of the stator windings. For example, thermal stress and/or voltage stress may lead to insulation breakdown, which in turn, may result in partial short-circuiting and/or open-circuiting of individual turns of the stator windings.
In order to diagnose the stator windings, some prior art techniques utilize high frequency voltage injection by injecting a high-frequency voltage on top of the fundamental excitation voltage and measuring the resulting stator currents in the negative sequence carrier-signal reference frame. However, difficulties arise for certain machine types. For synchronous machines with inherent saliency (such as interior permanent magnet or synchronous reluctance type), as the rotor speed approaches zero, all frequencies in the electric motor converge to zero, in which case, the frequency components corresponding to stator winding faults become indistinguishable from the normal motor current frequency components. Even for synchronous machines without an inherent saliency, such as a surface mount permanent magnet machine, secondary effects such as saturation often result in significant saliency which can cause the same effect described above. For these reasons, for synchronous machines the rotor must be rotating at some non-zero speed in order to force spectral separation between the negative sequence components attributable to stator winding faults and the components due to the saliency of the machine. For asynchronous machines, such as an induction machine, a similar situation can arise due to saturation induced saliency or slot harmonics, unless the induction machine has a particular combination of rotor slots, stator slots, and pole pairs, which does not produce any conflicting negative-sequence components in the carrier-signals at zero speed or stall condition.
Some prior art techniques employ pulse-width modulation to extract the zero sequence voltage which may contain information related to stator winding faults. Similarly, this technique is not suitable for zero speed or low speed operation, as the motor harmonics converge to zero. Other methods for Volts per Hertz controlled drives examine the stationary frame stator currents to identify current distortion. However, this technique relies on a rotating electric field to detect the current imbalance, and is not suitable for zero speed or low speed operation. Thus, the prior art techniques are generally ineffective for stationary motors during start-up conditions, i.e., at zero or low rotor speed. In these situations, to detect the presence of a fault condition, the potentially faulty electric motor must be started and/or operated to detect the fault condition, which is counterintuitive and undesirable.